1. Technical Field of the Invention
This invention relates generally to mixed signal circuits and more particularly to noise suppression of such mixed signals circuits.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, an analog to digital conversion stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The analog to digital conversion stage converts the analog filtered signals into digital signals. The data recovery stage recovers raw data from the digital signals in accordance with the particular wireless communication standard.
As is further known, the signal strength of inbound RF signals may vary from 10 dB to -90 dB. For the weaker inbound signals, having as large of a signal to noise ratio as possible enables the receiver to more accurately recover data embedded in the RF signals. As such, any circuitry of the transceiver that generates noise adversely affects the sensitivity of the receiver (i.e., the ability to accurately recover data from low signal strength inbound RF signals).
One source of noise in the transceiver is a clock that generates a digital clock that is used by many of the digital circuits within the transceiver. For example, the digital clock may be an 80 MHz clock that is used by the data recovery stage, the data modulation stage, the analog to digital conversion stage, etc. Such a digital clock generates harmonic tones at multiples of the fundamental frequency (e.g., 80 MHz). where at least one of the harmonic tones has a frequency at or near the frequency of the inbound RF signals. For instance, if the inbound RF signals have a frequency of 2.4 GHz, the 30th harmonic tone of the 80 MHz clock has a frequency at 2.4 GHz. Such harmonic tones are carried on the power supply lines of the transceiver and coupled into noise sensitive circuits of the receiver.
Typically, to minimize the energy of the harmonic tones of the digital clock, a large capacitor is coupled on the power supply lines of the digital clock circuit (e.g., from VDD to VSS). In addition, many of the noise sensitive circuits, such at the analog input circuitry of the analog to digital conversion stage will include large capacitors on its power supply lines to minimize the noise coupled thereon. While this helps minimize the noise on the power supply lines, the die size of the capacitors is significant, which adds to the cost of manufacturing radio frequency integrated circuits.
Therefore, a need exists for a method and apparatus of reducing noise associated with harmonic tones of digital circuitry using minimal sized components.